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Multilevel Inverter for Grid-Connected PhotovoltaicSystems with Active Filtering Function
Kleber Oliveira, João Afonso, Marcelo Cavalcanti
To cite this version:Kleber Oliveira, João Afonso, Marcelo Cavalcanti. Multilevel Inverter for Grid-Connected Photo-voltaic Systems with Active Filtering Function. 4th Doctoral Conference on Computing, Electricaland Industrial Systems (DoCEIS), Apr 2013, Costa de Caparica, Portugal. pp.289-298, 10.1007/978-3-642-37291-9_31. hal-01348766
Multilevel Inverter for Grid-connected
Photovoltaic Systems with Active Filtering Function
Kleber C. Oliveira 1, João L. Afonso
2; Marcelo C. Cavalcanti
3
1Department of Electrical Engineering, Federal University of Paraíba, João Pessoa, Brazil 2 Centro Algoritmi – University of Minho – Guimarães, Portugal
3Department of Electrical Engineering and Power Systems, Federal University of
Pernambuco, Recife, Brazil, 1 [email protected],
2 [email protected], 3 [email protected]
Abstract.This paper presents a single-phase inverter with multilevel topology
adopted to interface photovoltaic systems with the electrical power grid, and at
the same time, to eliminate harmonics currents and to compensate reactive
power, operating as shunt active power filter. Multilevel inverters have been
attracting increasing attention in the past few years as power converters of
choice in high voltage and high power applications. Various topologies to
implement these inverters have been introduced and studied recently. The
theory of instantaneous reactive power applied to compensation of single-phase
circuits is also presented in this paper. This theory is a particular case of the
well-known p-q theory, which was originally developed for three-phase
circuits. Simulation results demonstrate this control algorithm applied to
multilevel single-phase topologies.
Keywords: Photovoltaic System, Active Power Filter, Multilevel Inverters,
p-q Theory.
1 Introduction
There has been increasing interest in electrical power generation from renewable-
energy sources, such as photovoltaic (PV). In the last decades, solar energy has been
one of the most active research areas among renewable-energy sources. Photovoltaic
modules can be used in stand-alone or grid-connected applications [1]. While
photovoltaic systems connected to the electrical power grid are gaining more and
more visibility, demand for energy is increasing worldwide.
To interface photovoltaic modules with the electrical power grid it is necessary to
use a power electronics inverter, which has to perform two tasks: to inject a sinusoidal
current into the grid and to ensure that the PV modules operate at the maximum
power point (MPP).
Most of the topologies for PV systems are multi-stage, having a high frequency
transformer or line-frequency transformer that adjusts the inverter DC voltage and
isolates the PV modules from the grid. However, the conversion stages decrease the
efficiency and make the system more complex [2].
286 K. C. Oliveira et al.
2 Contribution to Internet of Things
The use of multilevel inverters for photovoltaic applications is becoming popular in
recent years. By using multiple levels on the DC bus, the stress on each switching
device is reduced. Researches related to multilevel inverters have been reported in
applications about FACTS (flexible AC transmission systems), active filters and grid
interface for non-conventional energy sources.
By synthesizing the AC output terminal voltage from several levels of voltages,
staircase waveforms can be produced, which approach the sinusoidal waveform with
low harmonic distortion, thus reducing filter requirements.
The fact that multilevel converters need several DC sources in the DC side makes
them attractive for photovoltaic applications.
In the last few decades, the proliferation of nonlinear loads, like switching power
supplies, adjustable speed drives and commercial lighting, has resulted in serious
degradation of power quality in the electrical grids, mainly because of harmonics
problems. Reactive power consumption by the loads is another significant problem to
the electrical power grids. Both harmonics and reactive power generate extra power
losses in the distribution system transformers and cables, reducing the overall
efficiency of the power system. Therefore, compensation of harmonics and reactive
power is very important in order to reduce costs due to power losses and bad
functioning of equipment in the consumers and in the electrical distribution system.
This paper presents an inverter with multilevel topology, which interfaces
photovoltaic systems with the electrical power grid, and that at the same time
compensates harmonics currents and reactive power. The presented solution can be an
interesting option to the future Smart Grids, contributing to the Internet of Things.
3 p-q Theory
The p-q theory was first introduced by Akagi [3], and is also known as the
instantaneous power theory. It is an interesting tool to apply to the control of active
power filters (APF), or even to analyze three-phase power systems in order to detect
problems related to harmonics, reactive power and unbalance.
The p-q theory applies a Clack transformation to the system voltages and currents,
which is represented by the matrices in (1). This transformation converts the voltages
and currents from the phases a-b-c to the α-β-0 coordinates. It has the advantage of
allowing separating the zero sequence components of the voltage (v0) and current(i0).
With the voltage and currents determined in the α-β-0 coordinates, can be
calculated the instantaneous real power (p), the instantaneous imaginary power (q),
and the zero sequence power (p0), according to (2).
The power components p and q depend only on the α and β coordinates of the
voltages and currents, and the power component p0 depends only on the 0 coordinate
of the voltages and currents.
In order to compensate reactive power, harmonics currents and current unbalance
(eliminating the neutral wire current), it is necessary to compensate the power
components q and p0, and the oscillating value of p ( ). The compensation reference
currents in the α-β-0 coordinates are obtained from (3) and (4), which come from the
inverse of (2), and where qx = q, and px is given by (6).
Multilevel Inverter for Grid-connected Photovoltaic Systems 287
∝ = 23 1 −12 −120 √32 −√321√2 1√2 1√2
.
∝ = 23 1 −12 −120 √32 −√321√2 1√2 1√2
.
(1)
= ∝ 0− ∝ 00 0 . ∝
(2)
In order to compensate reactive power, harmonics currents and current unbalance
(eliminating the neutral wire current), it is necessary to compensate the power
components q and p0, and the oscillating value of p( ). The compensation reference
currents in the α-β-0 coordinates are obtained from (3) and (4), which come from the
inverse of (2), and where qx = q, and px is given by (6).
∗ = (3)
!"∗∗ # = 1"$ + $ & ∝ − ∝' . &((' (4)
An active power filter with a control system that presents a fast response, like the
one based on the p-q theory, has basically two advantages [4]:
• It presents a good dynamic response, producing the correct values of
compensating current in a short time after variations of the power system
operating conditions;
• If the active filter has a fast control system, its energy storage element will suffer
less to compensate the power system parameters variation.
The calculations of the p-q theory also allow a simple method to adjust and
regulate the DC link voltage. For that it is necessary to calculate a regulation power
(preg), which can be obtained through a proportional controller, Kp. Thus: )*+ = K-./∗ − /0 (5)
Where:
Kp – proportional controller; /∗ – DC link reference voltage; / – DC link voltage.
The regulation power, besides regulating the DC link voltage, can be used to set
the value of power to be injected in the electrical grid, produced by the solar
photovoltaic system. The regulation power is included in the instantaneous real power
value to be compensated, so that px is given by: ( = 1 − + )*+ (6)
288 K. C. Oliveira et al.
The p-q theory is a theory for three-phase electrical systems. An easy way to apply
this theory to single-phase systems consists in creating virtual voltages and currents
for the two non-existing phases, delaying them by +120º and +240º in relation to the
existing phase voltage and current.
In Fig. 1 is presented a block diagram of the proposed control system.
Fig. 1. Block diagram of the control system.
4 Multilevel Inverters
The term multilevel was introduced by Nabae et al. [5], and their structures were
developed from three-level inverters. These inverters have the ability to operate at
high voltage levels using devices with low power voltages, producing low harmonic
content at the output waveforms, when compared with the waveforms obtained with
two-level inverters.
In the Multilevel Inverters topologies, when the number of levels increases, the rate
of total harmonic distortion (THD) of the output voltage decreases. However, the
number of power devices also increases, increasing the complexity and cost of the
inverter.
The main topologies of Multilevel Inverters are the Neutral Point Clamped (NPC)
Inverter, the Flying Capacitor Inverter and the Cascaded H-Bridge Inverter.
Neutral Point Clamped Inverter
The output voltages of this topology have a lower harmonic content, enabling the use
of semiconductor devices in half the threshold voltage of the devices used in the two-
level topologies.
Multilevel Inverter for Grid-connected Photovoltaic Systems 289
The three-phase three-level NPC inverter has a function of transferring energy
from the source at the DC side to the AC side, or acting as a rectifier, transferring
energy from the AC side to the DC side. This property of reversibility, usually present
in other multilevel converters, requires two circuits connected to the inverter that are
able to provide and/or receive this energy.
Flying Capacitor Inverter
Although the Flying Capacitor Inverter (FCI) topology is not as common as other
structures, it has some distinct advantages over the diode-clamped topology, including
the absence of clamping diodes and the ability to regulate the flying-capacitor
voltages through redundant state selection, even if the number of voltage levels is
greater than three.
Unlike the NPC inverter topology, the Flying Capacitor Inverter has redundant
switching states for synthesizing the phase voltage, so that some phase voltage vectors
can be synthesized by more than one switching state [6].
Cascaded H-Bridge Inverter
This topology consists of two or more single-phase full bridge converters with their
outputs connected in series at the AC side. Thus, each converter creates three different
levels of voltage. The major advantages of the Cascaded H-Bridge converters are
scalable power rating, modularity, and cost effectiveness. The output voltage of the
Cascaded H-Bridge converter is the summation of the output voltage of the individual
H-bridges. By increasing the number of series H-bridge converters, the output voltage
of the converter can be increased, while the switching frequency of the individual H-
bridge converter can be decreased to achieve the same output waveform quality [7].
After studying the three basic topologies of the multilevel inverters (Neutral Point
Clamped - NPC, Cascaded and Flying Capacitor), the NPC topology was chosen
mainly due to its characteristics of easy control, especially when compared to the
Flying Capacitor topology that has a difficult voltage control of the clamping
capacitors [8][9]. The Cascaded H-Bridge inverter needs separate DC sources for real
power conversions, and thus its applications are somewhat limited. The Flying
Capacitor Inverter control would be very complicated, and the switching frequency
and switching losses would be high for real power transmission [10].
5 Simulations Results
The proposed control system, with function of active power filter using the p–q
theory, is applied to a single-phase NPC inverter connected to the electrical power
grid. Simulations were performed in Matlab/Simulink for different loads: linear and
non-linear. The linear load is composed by one inductance (1 mH) in series with one
resistance (5 Ω). The non-linear load is composed by a single-phase rectifier with
C = 640 µF in parallel with R = 5 Ω at the DC side.
The simulations conditions are the following: switching frequency Ts= 4 kHz,
source voltage vs = 230 V (RMS value), source frequency fs= 50 Hz, source
290 K. C. Oliveira et al.
inductance Ls = 1mH, link DC voltage Vdc = 700V, and DC link capacitors
C1 = C2 = 4.7 mF.
In order to compare the behavior of the output filter currents and the balance of the
DC link voltages of the NPC inverter, simulations were performed with two different
PWM techniques:
• In-Phase Disposition PWM (IPD PWM);
• Phase Opposition Disposition PWM (POD PWM).
Booth techniques require each of the 2 carrier waveforms for 3-level phase
waveform. In the POD PWM the carrier waveforms are all in phase above and below
the zero reference value, however, there is 180 degrees phase shift between the ones
above and below zero respectively.
The IPD PWM technique, generally, is widely used in multilevel inverters. This
technique is similar to the previously described one, except that all carriers are in
phase. The zero reference is placed in the middle of the carrier set.
The Flying Capacitor inverter presents difficulties in controlling the balancing of
the voltage of the clamped capacitors and of the flying capacitor, too, for booth PWM
techniques. One problem is that an excessive number of storage capacitors are
required when the number of converter levels is high. The Cascaded H-Bridge
inverter and the NPC inverter present good results in balancing the DC link voltage.
But the Cascaded inverter needs separate DC sources for real power conversions, and
thus its applications are somewhat limited. Reactive power flow can be controlled more
easily in the NPC inverter. The NPC inverter presents the best behavior among the multilevel inverters. Using
the IPD PWM, the NPC inverter has a lower THD grid current. Therefore, the NPC
inverter with the IPD PWM was chosen to be simulated with the two loads.
In order to validate the control system for the NPC topology, the single-phase NPC
leg connected to the grid was tested as an inverter. In fact, a three-phase NPC
topology can be obtained using three independent single-phase inverters, like the one
shown in Fig. 2, connected through the common neutral.
Fig. 2. Single-phase NPC inverter connecting the PV system to the electrical power grid.
The grid voltage and current, the reference current, the filter current and the load
current are shown in the Fig. 3 and Fig. 4, respectively for a linear and non-linear
Multilevel Inverter for Grid-connected Photovoltaic Systems 291
load. For booth loads the NPC inverter presents the best current control among the
studied multilevel topologies. The control of the DC link presents good results for the
NPC and the Cascaded inverters.
6 Experimental Results
In order to obtain results for the single-phase NPC inverter topology with active
power filter function, it was developed an experimental laboratory prototype. The
complete system control was executed in discrete time using the TMS320F2812 DSP
from Texas Instruments, working with sampling and switching frequencies of 10 kHz.
The conditions of the experimental results are the same of the simulation results,
except that the adopted experimental grid voltage was 115 V. The two loads
conditions are the same, too.
Fig. 3. NPC inverter with linear load using a PWM technique IPD.
Fig. 4. NPC inverter with non-linear load using a PWM technique IPD.
292 K. C. Oliveira et al.
Fig. 5 shows the results for operation with the linear load, without and with active
power filter function, in which, for this case, the inverter compensates reactive power.
In Fig. 5(a) the active filter function is off, and the grid current (is) is delayed almost
90o in relation to the grid voltage (vs). The amplitude of the reference current (i
*f) is
close to the amplitude of the load current (ic), since the load current should be almost
completely compensated. However, the filtering current is zero, because the active
filter function is off. In Fig. 5(b) the active filter function is on, and the filtering
current (if) follows its reference current. As result, the grid current (is) has its
amplitude reduced, and it becomes in phase with the grid voltage.
(a) (b)
Fig. 5. Experimental results of the single-phase NPC inverter with linear load:
(a)Without active filtering; (b)With active filtering.
(a) (b) Fig. 6. Experimental results of the single-phase NPC inverter with non-linear load:
(a)Without active filtering; (b)With active filtering.
Multilevel Inverter for Grid-connected Photovoltaic Systems 293
Fig. 6shows the experimental results for operation with the non-linear load (single-
phase rectifier with RC parallel load), without and with active power filter function, in
which, for this case, the inverter compensates mainly harmonics currents. Fig. 6(a)
shows the waveforms when the active filter function off. The grid current (is) is
distorted but almost in phase with the grid voltage (vs). In Fig. 6(b) the active filter
function is on, the filtering current (if) follows the reference current (i*f),
compensating the harmonics currents, and the grid current becomes sinusoidal.
7 Conclusions
This paper presents a single-phase multilevel NPC (Neutral Point Clamped) inverter
with active filtering function, to be used to interface photovoltaic systems with the
electrical power grid, and at the same time, to eliminate harmonics currents and to
compensate reactive power, operating as shunt active power filter.
The control system is based on the p-q theory, proposed by Akagi, adapted to work
in single-phase electrical systems. This control theory permits the compensation of
harmonics currents and reactive power, and the setting of the power produced by the
photovoltaic system, to be injected by the inverter in the electrical power grid.
Three multilevel inverters topologies are presented, and the choice of the NPC
topology is explained.
The single-phase multilevel NPC inverter with active filtering function is tested,
and are presented both simulation and experimental results, obtained with a laboratory
prototype. Simulations and experiments were performed with two types of load,
corresponding to different active filtering conditions: linear load, with reactive power
compensation; non-linear load, with harmonics currents compensation.
The studied multilevel inverter presented satisfactory performance for the cases
here evaluated, showing that the combined operation of active filtering and
interfacing of a photovoltaic system with the electrical power grid can be an
interesting possibility.
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